The present invention relates to a sample servo type optical information medium capable of recording and reproducing information optically, the information medium indicating a replica plate or a stamper or master for fabricating a replica plate.
As tracking servo methods adoptable in an optical disc capable of recording and reproducing information there are two methods which are a continuous groove servo method and a sample servo method. The sample servo method is described, for example, in U.S. Pat. No. 4,402,061.
Further, as a patent application relating to sample servo control there has been filed Japanese Patent Application No. 62-42522 (Feb. 27, 1987), which has also been filed in the U.S. and Europe.
The continuous groove servo method has long been developed, while the sample servo method has recently been developed actively because of high tracking stability.
According to the sample servo method, sector address portions and sample mark areas are disposed in places along an imaginary track beforehand on a replica plate of an optical disc. It is necessary that about 30 sector address portions be present on one circle of the imaginary track and 1,000 to 3,000, usually 1,376, sample mark areas present thereon.
And in each sample mark area there are present two sample marks and a clock pit, as a pair, along the center line of the imaginary track.
The sample marks are formed as wobble pits wobbled symmetrically right and left from the center line of the imaginary track. The clock pit is formed so that the center thereof is positioned on the center line of the imaginary track. The sample marks will hereinafter be referred to as wobble pits.
These wobble and clock pits each have a length, t, of 90 nsec on time base. For example, the pit length, t, is 0.5 .mu.m at a disc radius of 30 mm and a disc revolution of 1,800 rpm. An optical depth of each pit is .lambda./4 (.lambda.represents the wavelength of laser light used in information recording and playback, usually 830 nm).
For recording or reproducing information using such sample servo type optical disc, laser light is radiated onto the replica plate from a recording/playback head (not shown), then the reflected light is detected and a tracking control is performed for the recording/playback head so that the head occupies a position in which the quantities of reflected lights from the wobble pits are the same. By so doing, the recording/playback head can trace over the center line of the imaginary track passing through the center of the clock pit. And the detection of clock data is effected by detecting the quantity of reflected light from the clock pit.
Of most importance in the sample servo type optical disc is in which positions wobble pits are to be formed. If the positions where wobble pits are formed are not symmetrical right and left with respect to the center line of the imaginary track passing through the center of the clock pit, the wobble pits will become different in size (shape) from each other under the wobble pit forming method available.
That the wobble pits are not in positions symmetrical right and left with respect to the center line of the imaginary track and are different in size (shape) from each other, means that it is impossible to effect an accurate tracking control.
Therefore, during the optical disc fabricating process, it is a very important matter to measure the wobble width (wobble quantity) of each wobble pit, namely, the distance from the center line of the imaginary track to the center of each wobble pit, to estimate the condition of each wobble pit being formed.
In the prior art, however, there are present only pits on the sample servo type optical disc, so for measuring the wobble width of each wobble pit referred to above, there is no other way than measuring it with reference to the clock pit through the center of which the center line of the imaginary track passes. However, since there is a considerable distance between each clock pit and each wobble pit, it is difficult to even judge which clock pit and wobble pit are in a pair. Thus, it has been very difficult to actually measure the wobble width of each wobble pit.